1. Field of the Invention
This invention relates to a light source used for an optical pickup apparatus capable of optically accessing an optical disc, and more particularly to a variable light source capable of varying the size of light beam. Also, this invention is directed to an optical pickup apparatus for a different-type optical disc which can simplify its configuration by employing the variable light source.
2. Description of the Prior Art
The recent recording medium requires a capability of recording a higher capacity of information in accordance with change into a large-scale of information quantity. Accordingly, an optical disc showing a remarkable increase of an occupation rate in the recording medium market also has been developed such that it is possible to record a high capacity of information. As a result, in the optical disc market, there has appeared a digital versatile or video disc (DVD), a recording capacity of which is much greater than that of the existing compact disc(CD) and the rewritable compact disc (CD-R). This DVD is not only higher than the CD and CD-R in a recording density, that is, in a track density, but also it is shorter than those in a distance extending from the disc surface to the information recording face. In reality, the distance from the disc surface to the information recording face in the DVD is 0.6 mm while that in the CD and CD-R is 1.2 mm. Due to this, the wavelength .lambda. and the beam size BS of a light beam, and the numerical aperture NA of an objective lens in an optical pickup apparatus for the DVD become different from those in an optical pickup apparatus for the CD or CD-R. For example, the wavelength .lambda..sub.c and the beam size BS.sub.c of a light beam used for the optical pickup apparatus for the CD or CD-R must have 780 nm and 1.4 .mu.m, respectively, and the numerical aperture NA.sub.c of the objective lens must maintain a range of 0.35 to 0.25. On the other hand, the wavelength .lambda..sub.D and the beam size BS.sub.D of a light beam used for the optical pickup apparatus for the DVD must have, 635 to 650 nm and 0.9 .mu.m, respectively, and the numerical of aperture NA.sub.D of the objective lens must maintain 0.6.
In order to access all of the CD, CD-R and DVD, the optical pickup apparatus must be able to vary the size of light beam. However, the conventional light source could generate a fixed size of light beam only because it basically takes a structure incapable of varying the size of light beam. This results in the conventional optical pickup apparatus using two light sources as well as having the complex structure.
In reality, as shown in FIG. 1, the conventional light source includes an n-type AlGaAs glut layer 12, a multiple quantum well active layer 14, a p-type AlGaAs glut layer 16 and a P-type GaAs contact layer 18 which are sequentially disposed on an n-type GaAs substrate 10 in such a manner to have a double hetero structure. The conventional light source further includes an n-type GaAs block layer pattern 20, a p-type GaAs buried layer 22 and a conductive layer 24 which are sequentially formed on the upper surface of the p-type GaAs contact layer 18 in such a manner to make a current confined structure. As shown in FIG. 2, the n-type GaAs block layer patterns 20 are positioned at both the left and right sides to provide a guide hole 21 with a tunnel shape in the middle thereof.
In the conventional light source having the structure as described above, if a certain amount of exciting current Im is applied to the conductive layer 24, then holes drift from the p-type GaAs buried layer 22, via the p-type GaAs contact layer 18 and the p-type AlGaAs glut layer 16, into the multiple quantum well active layer 14. Also, electrons drift from the n-type semiconductor substrate 10, via the n-type AlGaAs glut layer 12, into the multiple quantum well active layer 14. A laser light beam is generated in the multiple quantum well active layer 14 by holes entering from the upside thereof and electrons entering from the downside thereof. At this time, the laser light beam is generated in the region positioned at the bottom side of the guide hole 21 in the multiple quantum well active layer 14, and has a constant beam size. This results from holes drifted by the exciting current being uniformly distributed in the guide hole 21 and being not drifted in a plane direction in the guide hole 21.
As described above, the conventional light source is unable to control the size of light beam because it does intrinsically take a structure generating a constant size of light beam. Due to this, the conventional different-type optical disc pickup apparatus had no choice but to use two light sources.